Process for manufacturing enhanced thermal conductivity oxide nuclear fuel and the neclear fuel

ABSTRACT

A nuclear fuel and a method to produce a nuclear fuel wherein a porous uranium dioxide arrangement is provided, the arrangement is infiltrated with a precursor liquid and the arrangement is thermally treated such the porous uranium dioxide arrangement is infiltrated with a precursor liquid, followed by a thermal treating of the porous uranium dioxide arrangement with the infiltrated precursor liquid such that the precursor liquid is converted to a second phase.

FIELD OF THE INVENTION

The present invention relates to fuel for nuclear reactors. Morespecifically, the present invention provides a nuclear reactor fuel anda process for making a nuclear reactor fuel which exhibits enhancedthermal conductivity as compared to conventionally used uranium dioxidenuclear reactor fuel.

BACKGROUND INFORMATION

Present-day nuclear power reactor fuels in use for commercial powergeneration are based on uranium dioxide. The uranium dioxide fuel iscommonly a product of several manufacturing steps including pressing auranium dioxide powder into a pellet shape and subsequently firing thepellet to remove any formed voids.

The wide-spread use of uranium dioxide fuel is due primarily to the manydesirable characteristics of the uranium dioxide material, including ahigh density of uranium atoms necessary for producing a nuclearreaction, inertness and insolubility of the uranium dioxide in hightemperature water, a high melting point and an absence of neutronpoisons which could affect reactor performance. Although uranium dioxideis satisfactory for use in light water reactors, uranium dioxide alsohas several significant drawbacks which limit its overall effectiveness.Chief among the drawbacks is a relatively low thermal conductivity ofuranium dioxide which imposes significant limitations on reactoroperations. The inability of uranium dioxide to remove large quantitiesof heat effectively limits overall reactor operation and alsocompromises reactor operations during transient events such as loss ofcoolant accidents (LOCA). The nuclear industry has made attempts toincrease thermal conductivity of uranium dioxide fuel, but none of theattempts have been successful. Despite the drawbacks, uranium dioxide,in unmodified form, remains the dominant fuel for nuclear powerreactors.

In general, heat produced in nuclear fuel must be conducted through thebody of the fuel, normally in the pelletized form, and an externalcladding, normally a zirconium alloy, to a surrounding coolant layer inorder to properly cool the fuel and prevent pellet degradation. Thesurrounding coolant layer is moved past the external cladding to providea consistent temperature for removal of heat from the fuel. Duringtransient reactor conditions, such as when the coolant flows past theexternal cladding unevenly, the steady removal of heat from the pelletis disrupted. During loss-of-coolant accidents, operational safety iscompromised due to accumulating heat in the fuel and the inability ofthe uranium dioxide matrix to withstand the increased temperatures. Thisthermal conductivity characteristic of conventional uranium dioxide fuelnecessitates operating the reactor at reduced power in order to achieveacceptable overall plant safety margins. Operating the reactor at thereduced power levels consequently affects overall plant operating costs.Current nuclear fuels using uranium dioxide also have a limited burn-upcapacity. The limited burn-up capacity reduces the overall costeffectiveness of the fuel. The limited burn-up capacity results fromgreater fission gas release inside the fuel cladding over time. Thegreater fission gas release thereby results in higher fuel rod internalpressure, potentially leading to cladding deterioration due to thehigher stress. The elevated temperatures of the existing fuel alsostresses the fuel cladding thereby limiting overall service life.

There is a need to provide a nuclear fuel which will provide enhancedthermal conductivity compared to conventional uranium dioxide fuelcurrently used in nuclear power reactors.

There is a further need to provide a nuclear fuel which will result ingreater safety of the nuclear reactor under accident conditions, such asloss of coolant accidents.

There is a still further need to provide a nuclear fuel which willpossess superior burn-up capabilities compared with conventional uraniumdioxide nuclear fuels, thereby allowing greater fuel utilization,improved economy, and limited fission gas release.

SUMMARY

It is an object of the present invention to provide a nuclear fuel whichprovides enhanced thermal conductivity compared to conventional uraniumdioxide nuclear fuel currently used in nuclear power reactors.

It is also an object to provide a nuclear fuel which will result ingreater safety of the nuclear reactor under accident conditions, such asloss of coolant accidents.

It is furthermore an object of the present invention to provide anuclear fuel which will possess burn-up capabilities superior to that ofconventional uranium dioxide fuels, thereby allowing greater fuelutilization and limiting fission gas release.

These and other objects of the present invention are achieved asillustrated and described. The invention provides a method to produceuranium dioxide fuel which has increased thermal conductivity comparedto conventional nuclear fuel. The method recites providing a porousuranium dioxide arrangement, infiltrating the porous uranium dioxidearrangement with a precursor liquid, and thermally treating the porousuranium dioxide arrangement with the infiltrated precursor liquid suchthat the precursor liquid is converted to a second phase.

The invention also provides a nuclear fuel. The present inventionrecites an arrangement having a matrix of uranium dioxide and siliconcarbide interspersed in the matrix of uranium dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic representation of a process for making afuel assembly.

DETAILED DESCRIPTION

The present invention provides a nuclear fuel and a method to make thenuclear fuel. Referring to FIG. 1, a uranium dioxide arrangement 10 isprovided for processing. The uranium dioxide arrangement 10 may be inany shape, such as a pellet, ball or rod for example. The uraniumdioxide arrangement 10 should have a porous matrix to allow infiltrationof material into the arrangement 10 when contacted by a precursor liquid12. The porous matrix of the uranium dioxide arrangement 10 can beformed, for example, by pressing uranium dioxide powder into a “green”or unfired shape. The porous matrix may also be formed by a bisquefiring that does not fully densify the uranium dioxide arrangement 10.

A liquid precursor 12 is added to the uranium dioxide arrangement 10 toinfiltrate the uranium dioxide matrix. The precursor liquid 12 may be,for example, allylhydridopolycarbosilane (AHPCS). The precursor liquid12 can be configured to penetrate the porous matrix of the uraniumdioxide arrangement 10 without damaging the overall uranium dioxidematrix configuration. The uranium dioxide arrangement 10 may be sprayedor, as illustrated, immersed in the liquid precursor 12, to causecontact between the arrangement 10 and the liquid precursor 12. Thecomposition of the precursor liquid 12 allows incorporation of theprecursor liquid 12 into the pores of the porous matrix of the uraniumdioxide arrangement 10. The time of contact of the precursor liquid 12to the uranium dioxide arrangement 10 may be chosen such thatincorporation of the precursor liquid 12 into all of the pores in theuranium dioxide matrix occurs in a single contact cycle, for example. Topromote infiltration of the precursor 12 into the arrangement 10, thearrangement may be evacuated before infiltration, or the precursor maybe applied under pressure, or both. Alternatively, the precursor liquid12 may contact the uranium dioxide arrangement 10 for a time such that asingle contact cycle does not result in incorporation of the precursorliquid 12 into all of the pores of the porous matrix.

After the precursor liquid 12 has contacted the uranium dioxidearrangement 10 and been incorporated into the matrix of the arrangement10, at least partially, the arrangement 10 may then be cured. Curing 14may be through placement of the uranium dioxide arrangement 10 into afurnace 16 between, for example, 180 degrees centigrade and 400 degreescentigrade. Curing time may be, for example, between 2 hours and 8hours. Other curing times and temperatures may be used. The curing 14process converts the precursor liquid 12 into a solid polymer, whereinthe solid polymer is positioned in the matrix of the uranium dioxidearrangement 10.

Next, the arrangement 10 is then thermally treated 18 such that thepolymer positioned in the uranium dioxide arrangement 10 is converted toa second phase. In the current exemplary embodiment of the invention,the allylhydridopolycarbosilane, which has turned into a polymer in theuranium dioxide arrangement 10 from the curing operation, is convertedinto silicon carbide through firing the arrangement 10 in a furnace 20.The furnace temperatures may be chosen, for example, from between 800degrees centigrade to 1700 degrees centigrade. The residence time forthe uranium dioxide matrix in the furnace 20 may be, for example, 2hours to 8 hours. Other residence times may be used such that thepolymer is converted into silicon carbide. Residence times may be variedto minimize ultimate volume change of the pellet. The resulting productis a nuclear fuel which has silicon carbide incorporated into the matrixof the uranium dioxide.

The method steps of infiltrating the porous uranium dioxide arrangement10 with a precursor liquid 12 and thermally treating the porous uraniumdioxide arrangement 10 with the infiltrated precursor liquid 12, whichcan include both the curing and the firing of the arrangement, may thenbe repeated, if desired, to allow more incorporation of precursor liquid12 into the matrix of the uranium dioxide if total incorporation has notoccurred.

The present invention provides an increase in the thermal conductivityof nuclear fuel thereby resulting in increased fuel performance duringloss of coolant accidents. The present invention also provides forreduced fuel temperatures and internal fuel pellet heat. Due to thepossibility of creating overall geometric sizes similar to thegeometries used in conventional reactors, existing nuclear powerreactors may utilize fuel described in the present invention.Furthermore, through the use of the nuclear fuel with increased thermalconductivity, existing reactors may be operated at higher power levelsto provide superior economic performance. Maximum fuel burn-up is alsoincreased as lower overall fuel temperatures limit fission gas release,thereby limiting fuel rod internal pressure. Superior fuel burn-up alsoallows less waste to be produced for ultimate disposal. The reduced fueltemperatures also reduce the stresses imposed on the cladding, reducefuel cracking and relocation and reduce life-limiting fuel swelling.

Use of silicon carbide is compatible with existing light-water reactors,thermally, chemically and neutronically. New reactor systems, therefore,do not have to be created in order to utilize a fuel containing siliconcarbide. The thermal conductivity of silicon carbide is high so thatsubstantial increases in overall fuel thermal conductivity can beachieved with only a small decrease in the density of uranium atoms. Asan example, a thermal conductivity of 50 percent is expected for a 10percent volume loading of silicon carbide.

Advantages for a process of producing a nuclear fuel using siliconcarbide incorporated into the matrix of the arrangement include thelimited addition of an infiltration station and an inert-gascuring/firing furnace with provision for combustion of hydrogenoffgassed from the precursor to existing facilities used for productionof nuclear fuel. The process of the current application also allows theprecursor liquid to penetrate the entire body of the fuel arrangement sothat the resulting second phase, after thermal firing, penetrates to thecenter of the pellet, thereby producing a uniform overall fuel product.The second phase of the invention may form as a continuous networkrather than as discontinuous particles, so the overall fuel pellet iseffective in conducting heat from the core of the fuel pellet to theexterior surface. Additionally, the liquid infiltrant produces a highyield of the second phase, so the infiltration and conversion processneed only be repeated a few times.

The current invention provides an advantage over processes mixingpowders of uranium dioxide and silicon carbide in that for small volumefractions, the second phase forms discrete particles which are thermallyinsulated by the uranium dioxide. For large volume fractions, anexcessive amount of uranium is displaced affecting overall fuelcomposition.

The current invention also provides an advantage over chemical vaporinfiltration for placement of silicon carbide on uranium dioxide. Suchchemical vapor infiltration methods produce uneven placement of siliconcarbide on an exterior of a fuel element with higher concentrations ofsilicon carbide on the exterior of the fuel. Costly equipment is neededfor deposition of the chemical vapor on the uranium dioxide. Placementof the chemical vapor is also uneven, resulting in a final productwidely varying composition. Additionally, methyltrichlorosilane, used inthe deposition of silicon carbide, results in hydrogen chloride gasproduction as a waste product, thereby complicating waste disposalissues and increasing overall cost.

The current invention additionally provides advantages over mixinguranium dioxide with whiskers of silicon carbide. The whiskers, discretearrangements of silicon carbide, prevent effective sintering of thearrangement. Moreover, silicon carbide whiskers mixed with a uraniumdioxide powder would result in uneven silicon carbide distribution.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments, thereof. It will, however,be evident that various modifications and changes may be made thereuntowithout departing from the broader spirit and scope of the invention asset forth in the claims. The specification and drawings are accordinglyto be regarded in an illustrative rather than a restrictive sense.

1-16. (canceled)
 17. A nuclear fuel, comprising: an arrangement having amatrix of uranium dioxide; and silicon carbide interspersed in thematrix of uranium dioxide.
 18. The nuclear fuel according the claim 17,wherein the arrangement is pellet shaped.
 19. The nuclear fuel accordingto claim 17, wherein a total volume of the arrangement is comprised ofup to 10% silicon carbide on a volumetric basis.
 20. The nuclear fuelaccording to claim 19, wherein the silicon carbide is equallyinterspersed with the uranium dioxide.